This invention relates to the polymerization of 2-pyrrolidone and more particularly to the formation of pellets of this polymer by means of a continuous bulk polymerization process.
The polymerization of 2-pyrrolidone using an alkali metal salt of 2-pyrrolidone as a catalyst combined with a co-catalyst or "activator" under anhydrous conditions is well known in the art. Relative prior art is recited in our co-pending application Ser. No. 039,773. Most of the prior art disclosed methods of polymerization which are known as "bulk polymerization" resulting in the formation of an exceptionally hard and tough polymer cake which is very difficult to comminute. A method commonly employed is to grind the polymer cake in a Wiley mill or similar larger grinder. This is not only very expensive but the cake is so hard that it breaks particles of steel from the blades of the grinder thus introducing metal particles into the ground polymer which interfere with subsequent operations such as melt spinning through the tiny holes of a spinnerette to form fibers.
Some attempts to avoid this grinding step by forming pellets directly via bulk polymerization have been disclosed in the prior art. Thus U.S. Pat. No. 3,681,293 discloses a method of extruding the partially polymerized mass while still soft, cutting the extruded soft rod into pellets and then further polymerizing the pellets. U.S. Pat. No. 3,804,813 discloses a similar method in which the partially polymerized mass is cut into small pieces which are then further polymerized. Such methods in actual practice are virtually inoperable because it is difficult or impossible to carry out the various steps of the operation in the complete absence of moisture with the result that the further polymerization of the pellets does not occur since contact with even small amounts of water vapor stops the polymerization reaction. It is therefore essential that the bulk polymerization be carried out in a completely sealed apparatus until a conversion of 50 percent or more is reached before exposure to the atmosphere. Pellets having this polymer content are suitable for any subsequent use. The unpolymerized monomer may be recovered readily in the washing step which is necessary to remove the alkali. To facilitate washing it is desirable that the polymerization be stopped at about 50 to 60 percent rather than allow it to procede to maximum conversion.
Attempts have been made by us in the past to carry out the polymerization of 2-pyrrolidone continuously in small tubing completely sealed from the atmosphere using metal tubing coated with a low-friction material such as polytetrafluoroethylene ("Teflon") or by using Teflon or polyethylene tubing. The intention was to keep the polymerizing mass continuously moving through the tubing until a solid rod was formed which would emerge from the end. The difficulty encountered was that the residence time in the tubing was of necessity so long to reach even 40 percent conversion that the method became impractical. Not only were very long lengths of tubing required but the resistance to flow in such long lengths made the system inoperable. To make matters even worse, the polymerizing mass characteristically passes through a sticky plastic phase in the earlier stages of the polymerization which contributes greatly to the resistance to flow.
It is an object of this invention to provide a method of increasing the polymerization rate to such a degree that only relatively short lengths of tubing are required to provide the necessary residence time.
It is a further object of the invention to provide a method of minimizing the duration of the sticky plastic phase which normally occurs during the polymerization so that it is no longer a problem.
Other objects will be apparent in the detailed disclosure which follows.